1. Technical Field
The present invention relates to a thiophene-containing compound and a thiophene-containing compound polymer that are useful for organic electronic devices such as electrophotographic photoreceptor, organic electroluminescent device, and organic transistor. More specifically, it relates to a thiophene-containing compound and a thiophene-containing compound polymer superior in charge-transporting and photoemitting properties. The invention also relates to an organic electroluminescent device using a particular charge-transporting polyester, a method of producing the organic electroluminescent device, and an image display medium using the organic electroluminescent device.
2. Related Art
Regarding electrophotographic photoreceptors, along with recent improvement in performance, organic photoreceptors are used more frequently in high-speed copying machines and printers, but current electrophotographic photoreceptors are insufficient in performance under the circumstance, and thus, there is an urgent demand for elongation of the lifetime. Although there are some charge transport layers in the currently mainstream low-molecular weight dispersion system that almost satisfy the requirements in electrical properties, but the layers still had disadvantages in that they were still lower in mechanical strength and susceptible to abrasion, because a low-molecular weight compound is dispersed in polymer.
Generally, a low-molecular weight charge-transporting material deposited by vacuum deposition is used in the organic electroluminescent device, but there is observed a phenomenon that a great amount of Joule's heat generated by operation at a high current density of several mA/cm2 often causes morphological change of the low-molecular weight charge-transporting material, for example by crystallization, which in turn leads to disadvantages such as deterioration in luminescence brightness, dielectric breakdown, and consequent shortening of device lifetime.
There are various requirements in properties, such as solubility, film-formability, mobility, heat resistance, and matching of oxidation potential, demanded for the charge-transporting material, and thus, to satisfy these requirements, the physical properties thereof are generally modified by introducing a substituent. The physical properties of the charge-transporting polymer have close relationship with the physical properties of the raw material charge-transporting monomer, and thus, molecular design of the charge-transporting monomer, i.e., the low-molecular weight material, is important. The raw material monomers for the triarylamine polymer above include the following two monomers:
(1) Dihydroxyarylamine, and
(2) Bishydroxyalkylarylamine.
However, the dihydroxyarylamine (1) having an aminophenol structure is easily oxidized and thus, hard to purify. In particular, it is less stable when converted into a para-hydroxy substituted structure. Such a compound has a disadvantage that it may be lower in mobility because of uneven distribution of the charge caused by electron withdrawal by the oxygen atom due to the structure in which the oxygen atom is directly bound to the aromatic ring.
On the other hand, the bishydroxyalkylarylamine (2) is less influenced by the electron withdrawal by the oxygen atom because of the methylene group present in the molecule, but is harder to prepare the monomer thereof. Specifically, reaction of a diarylamine or a diarylbenzidine with bromoiodobenzene often gives mixed products because the bromine and iodine atoms are both reactive, resulting in lower yield of desirable product. In addition, the alkyllithium and ethyleneoxide used in replacing bromine with lithium have a problem that they are more hazardous and toxic and demand caution in handling.
Further, organic electroluminescent devices prepared with the π conjugation system polymer such as PPV or the polymer containing triphenylamine introduced on the polyphosphazene side chain described above had problems in color tone, light intensity, durability, and others.
For that reason, there exists a need for development of an organic electronic material that is easier to produce and superior in electric charge-transporting efficiency and emission characteristics, in development of organic electronic devices, such as organic electroluminescent devices having greater luminescence brightness and superior in stability during repeated use.
Electroluminescent devices (hereinafter, referred to as “EL devices”), which are self-luminous all-solid-state devices and are superior in visibility and resistant to shock, are expected to find wider application. Inorganic fluorescent materials are mainstream products currently, but these materials demand an AC voltage of 200 V or more for operation, and thus, have problems, for example, in high production cost and insufficient brightness.